tu ve hinh :
xet tamgiac KDA va tamgiac EDB co : DK = DE (gt)
DB = DA do D la trung diem cua AB (gt)
goc KDA = goc BDE (doi dinh)
=> tamgiac KDA = tamgiac EDB (c - g - c)
=> goc KAD = goc DBE (dn) ma 2 goc nay so le trong
=> KA // BC (dh) (1)
b, (1) => goc KAE = goc AEC (soletrong)
xet tamgiac KAI va tamgiac CEI co : goc KIA = goc EIC (doi dinh)
AI = IE do I la trung diem cua AE (gt)
=> tamgiac KAI = tamgiac CEI (g - c - g)
=> KI = IC (dn) ma I nam giua K va C
=> I la trung diem cua KC (dn)
vay_